The purpose of this three year grant is to examine the relationship between visual hallucinations and visual system abnormality in Parkinson's disease. Visual hallucinations are common symptoms and frequent causes of morbidity in Parkinson's disease, yet little is known about their etiology. Increasing evidence suggests that hallucinations in Parkinson's disease are not simply a medication effect, but are associated with the underlying disease process. Specificallly, evidence exists that suggest visual hallucinations in Parkinson's disease may be related to known visual system dysfunction in Parkinson's disease. In this study, thirty Parkinson's disease patients with visual hallucinations will be matched to thirty Parkinson's disease patients without visual hallucinations. They will be examined on neuropsychological tests assessing visual cognitive function, and will undergo visual evoked potentials. A subset of these patients (20 matched pairs) will also undergo functional magnetic resonance imaging (fMRI) to assess visual cortex function. It is hypothesized that Parkinson's disease patients with visual hallucinations will have greater evidence of visual system abnormality. Specifically they will demonstrate greater deficits of visual-cognitive function, greater latency on visual evoked potential and differences in activation of visual cortical regions on functional magnetic resonance imaging (fMRI) than those without visual hallucinations. It is hypothesized that these results will support a proposed biologic model of VH in PD regarding the role of dopamine abnormality in both the retina and basal ganglia that effect the regulation of function of visual cortex. The results of this study will increase knowledge regarding the neural mechanisms of visual hallucinations in Parkinson's disease and knowledge of visual system abnormality in Parkinson's disease. The results may also increase our understanding of visual hallucinations in other disorders. Conceivably, such knowledge could lead to strategies to prevent, minimize or treat such symptoms.